Of course, "height" references the height of the parent. But if it is not defined it's as good as nothing

I just tested it within a <div style="text-align:center; height:400px">, and it behaved as it should, even with the html {height:100}. I could even use a <p style="text-align:center; height:400px">. But of course, then the image dimensions would be fixed, and I don't want that.
Thank you, Jellby.